1. Field of the Invention
The present invention relates fuel cells with improved electrode durability.
2. Background Art
Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. In proton exchange membrane (“PEM”) type fuel cells, hydrogen is supplied as fuel to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. The oxygen can be either in a pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face and a cathode catalyst on the opposite face. The MEA, in turn, is sandwiched between a pair of porous diffusion media (“DM”) which in turn are sandwiched between a pair of non-porous, electrically conductive elements or plates which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell typically must be thin, chemically stable, proton transmissive, non-electrically conductive, and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cell stacks in order to provide high levels of electrical power.
With reference to FIG. 1, a schematic illustration of the normal operation and the processes occurring during either local H2 fuel starvation and start/stop induced degradation of a typical fuel cell stack with prior art MEAs is provided. Fuel cell 10 includes anode layer 12 and cathode layer 14 with polymeric ion conductive membrane 16 disposed between anode layer 12 and cathode layer 14. Currently, a normal cathode layer includes about 0.2 to 0.4 mg of platinum per cm2 with a typical cathode catalyst having approximately 20 to 60% weight Pt supported on a high-surface area carbon support (“x % wt. Pt/C”). Moreover, cathode layers are very porous in order to provide efficient access for the oxygen (from air) and H2 gas. In a typical cathode layer the volume fraction of polymer is about 20%, the volume fraction of carbon is about 20%, and the void volume is about 60%. Hydrogen (H2) is supplied to anode layer 12 via reactant flow-field channels 20 and through anode diffusion media 18. On the opposite side of fuel cell stack 10, oxygen (O2 usually from air) is fed to cathode layer 14. During normal operation, the hydrogen oxidation reaction (“HOR”) occurs at anode layer 12, and the oxygen reduction reaction (“ORR”) occurs at the cathode (see left-hand side of FIG. 1). The oxygen gets reduced to make water while the hydrogen gets oxidized to make protons. The protons traverse membrane 16, where they recombine with the oxygen to complete the overall reaction of the fuel cell of hydrogen plus oxygen to give water.
During the operation of such fuel cells, situations can occur occasionally which might cause corrosion of the catalysts' carbon-supports, either on the anode and/or the cathode. This carbon corrosion can arise for several reasons: 1) maldistribution of H2 between the many cells of a fuel cell stack under certain operating conditions can lead to understoichiometric supply of H2 to one of the cells (i.e., less H2 is supplied than is required to sustain the applied current), leading to a large increase of the anode potential (commonly referred to as “cell reversal”), accompanied by corrosion of the carbon-support of the anode catalyst (see, e.g., D. P. Wilkinson, J. St.-Pierre, in: Handbook of Fuel Cells—Fundamentals, Technology, and Applications (eds.: W. Vielstich, A. Lamm, H. A. Gasteiger), Wiley (2003): vol. 3, chapter 47, pg. 611); 2) maldistribution of H2 within a cell due to liquid-water induced blockage of anode flow-field channels and/or water-film formation within the anode diffusion medium (commonly referred to as “local H2 starvation”), leading to a local increase of the cathode potential due to O2 permeation through the membrane from cathode to anode, which in turn corrodes the carbon-support of the cathode catalyst (see, e.g., T. W. Patterson, R. M. Darling, Electrochem. Solid-State Lett. 9 (2006) A183); 3) formation of a H2/air front in the anode flow-field channel during fuel cell shutdown (i.e., air permeation into the H2-filled anode flow-field channels) or startup (i.e., H2 flow into air-filled anode flow-field channels) processes (commonly referred to as “start/stop degradation”), leading to localized increase of the cathode potential, which in turn corrodes the carbon-support of the cathode catalyst (see, e.g., C. A. Reiser, L. Bregoli, T. W. Patterson, J. S. Yi, J. D. Yang, M. L. Perry, T. D. Jarvi, Electrochem. Solid-State Lett. 8 (2005) A273).
Cell reversal occurs when understoichiometric amounts of H2 are applied to one or several cells of a fuel cell stack so that the overall fuel cell stack current cannot be sustained by H2 oxidation (H2→2H+2e−). In this case, electron/protons must be supplied by alternative reactions, namely by the carbon oxidation reaction (“COR”) and/or the O2 evolution reaction (“OER”) in the anode electrode:OER: H2→½O2+2H++2e− E0=+1.23 V vs RHE  (1)COR: C+2H2O→CO2+4H++4e− E0=+0.21 V vs RHE  (2)(where E0 is the potential referenced to the Reversible Hydrogen Electrode (RHE) potential)
Since the OER is negligible for Pt catalysts on conventional carbon supports (e.g., Pt/Ketjen Black and Pt/Black Pears listed in Table 1), this cell reversal leads to corrosion of the anode catalyst carbon-support according to Reaction (2), leading to rapid performance degradation of the MEA.
In the case of local H2 starvation in the anode electrode 12 (FIG. 1), O2 permeating from the cathode layer 14 (commonly referred to as “O2 crossover”) through the ion conducting membrane 16 is reduced in the region where H2 is depleted, leading to carbon-support corrosion of the cathode catalyst carbon-support in the cathode catalyst layer 14. As is shown in FIG. 1, this can occur by depletion of H2 in the anode flow-field channel 20 due to water blockage, such that region 20′ is still filled with H2 while region 20″ is depleted of H2. Alternatively, if a water film 22 is formed in the anode diffusion medium, H2 will be depleted in the anode catalyst layer 12 adjacent to the water film 22. Thus, significant corrosion of the carbon-support of the cathode catalyst will occur in the cathode catalyst layer 14 which is adjacent to region 20″ or 22. This leads to irreversible degradation of the MEA performance.
Start/stop degradation occurs in the presence of a H2/air front in the anode flow-field channels during either startup or shutdown of a fuel cell. This case is represented in FIG. 1 when region 20′ of the anode flow-field channels contains H2 while region 20″ contains air. Again, this leads to corrosion of the carbon-support of the cathode catalyst in the cathode catalyst layer 14 which is adjacent to region 20″.
There are a few system strategies that may help reduce the impacts of cell reversal, such as monitoring the cell voltage of each individual cell and reducing fuel cell stack power when reversal of the cell voltage of a single cell in the fuel cell stack is observed, but this and other measures will increase the total cost. Local H2 starvation caused by water blockage of anode flow channels and/or anode diffusion medium flooding may be relieved by special design of the flow field plates which is also costly. Also, U.S. Pat. No. 6,855,453 discloses a method of reducing carbon corrosion utilizing a catalyst to promote the reaction (2) and thereby simultaneously suppressing the carbon corrosion reaction (1). The implementation of graphitized carbon support (more corrosion-resistant) has been identified as one key solution to the carbon corrosion problem, which itself still needs a significant amount of developmental work and is currently in its infancy. However, such graphitized support strategies alone do not sufficiently reduce carbon corrosion rates under localized H2 starvation and start/stop conditions.
Accordingly, there exists a need for improved membrane electrode assemblies that have more tolerance to anode local H2 starvation and cell reversal.